There are known methods of EM survey, in particular a marine controllable source EM survey, based on controlled electric current excitation of a surveyed medium (methods of resistance to direct and alternative current), that allow to determine only one electrophysical parameter of the three listed above, namely, the electric resistivity, that is often not enough for exploration and delineation of the oil-gas deposits. The most popular method among them is referred to as a pulse alternative low-frequency current method, which is an establishing electrical field method.
According to field data, measured with respect to this method, an electric resistivity pr is calculated by means of a fundamental formula:ρτ=·ΔU/J,  (1)wherein:    J is a measured surge of electric current in a dipole electrical source;    ΔU is a measured voltage at the ends of receiving ground electrodes MN;    —is the geometrical factor of a probe device.    (referring to “Electric survey”, Geophysician Handbook, Tarkhov et al., Moscow, Nedra, 1980, p. 237 and pp. 422-406).
When using such an approach, traditionally applied for geoelectrical surveys with a controllable current source, only summarized data about elements of the geological formation structure can be obtained, in which structure the field is developed. The reason is that the spatial distribution of the current density J produced by the source is uncontrollable, and any information about this distribution in the real three-dimensional heterogeneous media is not available. This means that the normalization of measured electrical voltage ΔU by means of dividing it by source current intensity J, is senseless, since J carries no information about the surveyed medium, but reflects information about the power of the current source and the contact resistance of grounded electrodes of the dipole source.
Thus, the aforesaid resistivity methods are hardly applicable for exploration and delineation of oil and gas deposits, at least for two reasons: first, only one of the mentioned three electrophysical parameters of the surveyed medium needed for this purpose is registered; and second, the registered parameter is too rough, because it measures resistivity of the whole volume of the geological formation, in which the source's electrical field propagates. This happens due to the absence of any electric current focusing.
Due to specificity of the measuring during motion, marine EM surveys typically employ a symmetric measuring setup AMNB or a dipole axial setup ABMN.
Attempts of oil and gas deposits exploration were performed in Black Sea and in Barents Sea by “Soliton” and “Sevmorgeo” geophysical crews. Only the qualitative interpretation was performed. Signals were charted, and anomalous zones associated with oil and gas deposits were detected, according to an interpretation of the present inventors. However, signals of establishing field anomalies were sometimes observed, which reflected the presence of shallow gas deposits in rather simple geological environment, that is, on the Black Sea offshore. (Petrov, A. A., “Capacities of the electric field development method for offshore oil and gas exploration, Geophysics, No 5, 2000, EAGO, Moscow, p. 21).
For more complex geological environments, the anomalies of field establishing signals, obtained by AMNB and ABMN setups, are not necessary related to hydrocarbons deposits.
There exists a method of the marine EM survey that includes excitation of electromagnetic field in a surveyed formation by passing rectangular current pulses, with pauses between them, and measuring the first potential differences (voltages) at several distances from the excitation power source. (Belash, V. A., “A method of marine geological electric survey”, Patent of the U.S. Ser. No 1122998 from Mar. 6, 1983, Bulletin No 41, 1984). An essential disadvantage of this method is the absence of a focusing of the electric current that excludes the horizontal components of electric current density jx or jy in the observation point. This disadvantage does not allow solving the inverse problem by using the known solution of a mathematical physics equation for one-dimensional medium with sub-horizontal boundaries, which is needed for research of the natural heterogeneous three-dimensional media. For this reason, the inverse problems for this approach in the 3D heterogeneous media cannot be solved correctly, and the separation of the parameters, such as the electric conductivity and the induced polarization is dubious.